4x = 3y ⇔ x = $\frac{3}{4}.y$
7y = 5z ⇔ z = $\frac{7}{5}.y$
Ta có 2.x - 3.y + z = 6
⇔ 2.$\frac{3}{4}.y$ - 3.y + $\frac{7}{5}.y$ = 6
⇔ -$\frac{1}{10}.y$ = 6 ⇔ y = -60
x = $\frac{3}{4}.y$ = $\frac{3}{4}.(-60)$ = -45
z = $\frac{7}{5}.y$ = $\frac{7}{5}.(-60)$ = -84
